Resistance standard and method of making same



J. C. YAG ER Sept 17, 1957 RESISTANCE STANDARD AND METHOD OF MAKING SAME Filed Aug. -J. 8, 1955 f .wll. 3

INVENTOR.

J. (.YAGER ATTORNEY Uni d-sa s Patmto 2,806,930 RESISTANCE STANDARD AND METHOD or MAKING SAME Application August 18, 1955, Serial No. 529,163 2 Claims. (Cl. 201-74 This invention relates to an electrical resistance standard and especially to the construction of one Whose electrical resistance is a small fraction of an ohm.

An object of the invention is to provide such a resistance standard which contains few parts, is inexpensive to manufacture and has a long life.

A further object is to provide a resistor having opposed faces against which apair of current-carrying electrodes may be pressed to cause the passage of a constant current through the resistor and a pair of pins having ends projecting from said faces whereby the resistance may be determined by noting the voltage drop on a voltmeter whose terminals are connected to said pins.

Another object is to provide a resistor of the type described with a block of electrical insulation which has a guide face adapted to fit against andslide along one of said current-"carrying electrodes to thereby position the resistor correctly with reference to the electrodes.

Further objects will become apparent as the description of the resistance standard proceeds. For a better understanding of the invention reference is made to the accompanying drawing, in which:

Fig. 1 is a top view of a resistance standard embodying the invention;

Fig. 2 is a longitudinal sectional view taken on line 22 of Fig. 1;

Fig. 3 is an end view of the standard of Fig. 1;

Fig. 4 is a partly sectional and partly schematic view of the resistance standard in a testing apparatus and,

Fig. 5 is a sectional view similar to Fig. 2 also showing a modified form of electrodes used with the resistance standard.

The resistance standard comprises a single metal strip one end 11 of which is bent to provide upper and lower parallel leaves 12, 13. Strip 10 is preferably made of non-ferrous metal such as brass, bronze or other inexpensive metal which does not oxidize readily at room temperature. The particular metal selected will depend on the resistance the standard is to have, a metal of lower conductivity being used when a standard of higher resistance is desired. A drill is then passed through the leaves to provide aligned holes 14 in which short cylindrical non-ferrous metal pins 15, 16 are driven and secured by soldering or other means. The holes 14 are spaced from the bent end 11 of the strip by a distance sutlicient to provide a resistance in the portion of the strip between the pins equal to the desired value as, for example, 50, 100 or 200 microhms. A small conical recess 17 is cut in the center of each pin for a purpose to be described, thus leaving a fiat annular face 18 on the exposed end of each pin. The space between the leaves 12-13 is then filled with a mass 19 of plastic resin, preferably an epoxy or other resin which hardens at normal room temperature of about 70 degrees. A flat sheet 20 of Bakelite or other rigid insulating material is attached to leaf 12 by adhesive or other securing means (not shown). This sheet has a slot 21 extending therethrough whose end walls 22, 23 are semi-cylindrical to provide means for guiding the resistance standard along a cylindrical electrode 24. By having the center of pins 15, 16 fourteen millimeters from the bent end 11 of a brass strip 25 millimeters wide and approximately 1 millimeter thick, the resistance of the portion of the strip between the pins was approximately 50 microhms. When a number of standards of the same dimensions are made, the conductivity of the brass strip may vary somewhat and the resistance of some of the standards (as measured between the pins) may be slightly less than 50 microhm. Some metal may be ground ofl? one edge 25 of such strips to increase their resistance and bring it up to 50 microhms.

Fig. 4 shows a means for testing a resistance standard in which similar upper and lower electrodes are used each electrode 24 being made of copper and having a flat end face 26 adapted to press against strip 10. Electrode 24 has a threaded shank 27 on which a nut 28 is screwed. The electrode carries a spring 29 which has a pin 30 attached near its end, the pin having a pointed end adapted to seat in recess 17. Spring 29 has a circular hole which is of larger diameter than shank 27 and a rubber O ring 31 is interposed between the shank and spring. A pair of insulating washers 32 serve to retain the spring in position upon the tightening of nut 28. The shank 27 of upper electrode 24 is screwed onto the threaded end of a piston rod 33 whose upper end carries a piston 34 slidable in cylinder 35. Compressed air is supplied either above or below the piston by pipe 36 or 37, frorna source (not shown). Current is supplied to electrodes 24 by the circuit shown including adjustable resistor. 38 having a movable contact 39, battery 40 and ammeter 41. Springs 29 are connected to the terminals of a vacuum tube voltmeter 42 having a scale preferably graduated in microhms.

To use the standard, with the piston 34 raised and lower electrode 24 attached by means (not shown) to a stationary support 43, the resistance standard is inserted between the ends 26 of electrodes 24 and the standard manually raised with the curved end wall 23 of slot 21 sliding along the curved face of upper electrode 24 and with contact 30 disposed substantially over pin 15. When leaf 12 contacts the end face 26 of upper electrode 24, piston 34 is forced down by admitting compressed air above the piston from pipe 36. As the standard is moved down, the lower contact 30 enters the hole in pin 16 and lower leaf 13 is firmly pressed against the top face 26 of lower electrode 24. Contact 39 is then moved to give a predetermined reading such as 10.0 amperes on ammeter 41. If the resistance of the standard is correct (such as 50 microhms), meter 42 will give a reading of 50; if it is incorrect, its value will be indicated by the meter reading. After the reading is taken, the piston is raised by admitting compressed air to pipe 37 which causes upper electrode 24 to rise whereupon the resistance standard may be removed from between the electrodes.

The same apparatus may be used to measure the surface resistance between the faying surfaces of two aluminum sheets to be later spot welded together. Most airplane manufacturing companies require that such sheets be cleaned to remove any film of oil or dirt from their surfaces. The sufficiency of this cleaning is indicated by placing the overlapping sheets between the two electrodes 24, applying a known pressure to the sheets, adjusting contact 39 to give a predetermined reading, such as 10 amperes, on ammeter 41; and noting the reading on meter 42 which indicates the surface resistance in microhms.

Fig. 5 shows the resistance standard in use with a different type of electrode. In this case the similar electrodes 44 are hollow and each contains an insulating tube 45 within which is slidable a plunger 46 terminating in a cylindrical contact 47. The contact is normally pressed outwardly by a coil spring 48. A lead 49 is adapted to connect upper electrode 44 to the end of resistor 38 and a lead 50 to connect lower electrode 44 to the negative terminal of battery 40. A lead 51 is adapted to connect upper plunger 46 to one terminal of meter 42 and a lead 52 to connect lower plunger 46 to the other terminal of meter 42. A collar 53 applies pressure to upper electrode 44, the upper end of this collar being screwed onto the threaded end of piston rod 33. Lower electrode 44 is attached to a suitable support 54.

In using the resistance standard, it is placed between the separated electrodes 44 and raised with the curved end wall 22 of slot 21 resting against the cylindrical face of upper electrode 44. This guides the standard so that the fiat upper end of pin 15 becomes seated against the flat end of upper contact 47. Pressure is now applied to upper electrode 44 by piston 34, causing the standard to move down and the lower end of pin 16 to seat against lower contact 47. Further downward movement causes contacts 47 to move into the insulating tubes 45 and the leaf 13 to be pressed against the upper end 26' 'of lower electrode 44 while the lower end of upper electrode 44 is pressed against leaf 12 with a predetermined pressure. The resistance of the standard is then determined in the same manner as above described.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment of the invention is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Having thus described my invention, what I claim as new and useful and desire to secure by Letters Patent is:

l. A portable electrical resistance standard of known value, comprising a thin metal strip of uniform thickness throughout, said strip having a pair of parallel spaced apart leaves connected together by a bent strip portion; a mass of rigid electrical insulation bonded securely to said leaves and filling the space therebetween; a block composed of electrical insulating material secured to the exposed face of one of said leaves, the thickness of said block being substantially greater than that of said strip and said block having a large smooth-walled opening extending therethrough; and a pair of cylindrical metal pins secured to said leaves at points remote from the ends thereof, the axes of said pins being in alignment and the adjacent ends of said pins abutting against said mass of electrical insulation, said pins having plane exposed end faces spaced outwardly from said leaves and the end of one of said pins disposed within the opening in said block. 2. An electrical resistance standard whose resistance is less than 0.5 ohm comprising; a thin metal strip whose length is many times its thickness, said strip having two parallel spaced apart long fiat leaves connected together at one end by a bent portion of strip and said leaves having a pair of coaxial cylindrical holes passing therethrough at a substantial distance from the ends of the leaves; a pair of cylindrical metal pins whose inner ends are a driving fit in said holes and whose outer ends extend a substantial distance beyond the outer faces of said leaves and terminate in plane contact faces, each of said contact faces having a conical recess therein; and a mass of polymerized plastic insulation bonded by adhesion to said pins and leaves and filling the space therebetween.

References Cited in the file of this patent UNITED STATES PATENTS 2,261,667 Stroszeck Nov. 4, 1941 2,495,279 OBrien et a1 Jan. 24, 1950 2,500,605 De Lange et a1 Mar. 14, 1950 2,521,894 Brown Sept. 12, 1950 2,538,977 Mucher Jan. 23, 1951 2,600,485 Cox June 17, 1952 2,682,596 Cox et a1 June 29, 1954 

